This invention relates to a method and apparatus for cleaning, viewing, inspecting, and documenting the condition of underwater weldments employed at the junction of the various structural members of an offshore platform, particularly a deepwater platform on which said operations are not possible by the normal use of divers.
Present day offshore platforms used in the oil and gas industry are often formed of large diameter pipe elements in the form of three or more vertical or slanting legs interconnected or reinforced by cross-bracing tubular members. Such bottom-supported platforms have been used in waters as deep as 1025 feet. For example, one deepwater platform in the Gulf of Mexico in 1025 feet of water has eight legs that are made of 78 inch diameter pipe through which 72 inch diameter pipe piles were used. Between 2 and 3,000 cross-bracing members were used in the structure and since each member was welded at least twice to another member, this resulted in up to 6,000 weldments being formed in the platform. In addition, the platform is provided with over sixty 24-inch diameter vertical well conductors that are grouped near the center of the platform and through which individual wells are drilled. Further, there is secured to the platform a plurality of vertical pipe risers through which oil and gas may be separately pumped down to the ocean floor pipeline and thence to shore.
From a safety and operational standpoint, it is desirable to know the condition of the various weldments in the platform structure and to keep a record of them. Thus, it is desirable to periodically inspect the weldments and photograph them in order to keep a record of any corrosion that may be taking place or to know when one of the welded joints has cracked.
It is well known to employ remotely-controlled underwater vehicles operations at the end of a tether to observe and carry out operations on underwater structures such, for example, as an underwater wellhead as described in U.S. Pat. No. 3,099,316 and related U.S. Pat. Nos. 3,163,221-3,165,899 and 3,463,226. These patents are all directed to the use of a remotely-controlled underwater vehicle for use with an ocean floor wellhead. Underwater wellheads and pipelines located on the ocean floor have free space around them so that they may be approached easily by any type of a submarine or underwater vehicle without encountering any structure that would prevent their underwater television camera from viewing the structure of interest.
However, it is a different matter to use a tethered underwater vehicle that is remotely operated and move it around, over and between the various cross-bracing members of an underwater structure so as to clean, observe, and photograph in detail the thousands of welds in the structure. It is generally necessary to use a remotely-controlled underwater vehicle of small size so as to be able to move into areas of limited access. In addition, it is necessary to employ flexible or articulated arms on the underwater vehicle in order to be positioned in close proximity to some of the welds being studied, such, for example, as the welds around a diagonal brace at the point it meets horizontal and vertical members.